Rear Window Leaking

[cmart]

I have a 99 Silverado that I bought almost a year ago. I bought it from an individual that definitely hid some things from me. Although they are not big deal things, most of them are just small and annoying. Nothing that I can't overcome...because I absolutely love the truck.

The one thing that is driving me crazy is everytime I wash it or it rains...I've got to stretch out a beach towel across the extended cab seat to catch all the water coming in. It is a rear sliding glass, but it's coming through the hard plastic part that goes around the rear glass...not the glass itself. It leaks on the left and right sides at the top. I can see where the plastic is cracked and that's where the water is coming in. Is this piece available to buy by itself? Is it that simple to just take the outer plastic ring off and replace it? I'm dumbfounded on this one and need help.

Any help would be very much appreciated.

Thanks,
Cory